The present invention relates to deep section curing agent compositions for condenstion curable silicone elastomers.
In the prior art, condensation curable silicone elastomers, for example, compositions comprising a hydroxyl group-containing polyorganosiloxane and an alkyl silicate as the main components have been known to be curable within several hours to 24 hours at about room temperature with the use of a curing agent such as an organic tin compound. The cured products obtained exhibit excellent electrical properties, heat resistance and chemical properties, and therefore have been utilized widely for various kinds of applications such as for potting, for a master mold, and the like.
However, since curing of such condensation curable silicone elastomers proceeds gradually from the surface to the inner portion, it takes a disadvantageously long time to ensure curing to the bottom portion when the elastomer has a depth of several centimeters from the surface.
In order to overcome such disadvantage, there have been proposed a composition containing a polyorganosiloxane comprising (CH.sub.3).sub.3 SiO.sub.1/2 and SiO.sub.2 units, a polyorganohydrogen siloxane and a metal oxide (Japanese Patent Publication No. 11982/1971), a composition containing a water-soluble silane or siloxane (Japanese Patent Publication No. 19616/1972), a composition containing a water-soluble polyvalent alcohol or its derivative having hydroxyl groups (Japanese Patent Publication No. 8461/1973) and a composition containing glycerophosphoric acid or its metal salt (Japanese Patent Publication No. 23977/1976). However, these compositions involved disadvantages such as the additive added for curing the deep section had a deleterious effect on the stability of the silicone composition capable of being cured to an elastic state by addition of the curing agent (hereinafter called the silicone composition) or that curability of the deep section was not sufficient.
It has also been proposed to accelerate the curing speed by use of an organic tin compound in which hydroxyl groups are bonded to tin atoms such as hydroxylated dimethyltin oleate (Japanese Patent Publications Nos. 44983/1972 and 44984/1972), and this system is also known to be excellent in deep section curability. However, this curing agent involves the disadvantages that its manufacturing steps are complicated and gives products with qualities varying greatly from lot to lot, that stability is not good and, moreover, that there is a limitation in its applications due to its toxicity.
Accordingly, there has now been made a discovery in the method by which deep section curability can be imparted by the curing agent, while employing a tin compound conventionally used as the curing agent, and without impairing the stability of the silicone composition. That is, the curing agent compositions as disclosed in Japanese Patent Publication Nos. 951/1969 and 29878/1976 have been known. However, the curing agent composition of the former, comprising a tin salt of an organic carboxylic acid, water and a common solvent for these selected from alkanols and alkane phosphonates can be dispersed and mixed into the silicone composition with difficulty. On the other hand, the curing agent composition of the latter, comprising a lipophilic surfactant, water and an organic tin compound into a W/O type emulsion, cannot give sufficient deep section curability, because water can be formulated in a large amount with difficulty. For obtaining sufficient deep section curability, it is necessary to formulate a large amount of said curing agent composition, which, however, results also in an increased amount of the tin compound added, whereby there results the problem of extremely shortened working time. Further, when said composition is stored over a prolonged time, separation will occur between the water and the tin compound to give the disadvantage that homogenization must be conducted again.